/*
 * Copyright 2002-2020 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.http.codec.json;

import java.lang.reflect.Type;
import java.util.Map;

import kotlinx.serialization.KSerializer;
import kotlinx.serialization.SerializersKt;
import kotlinx.serialization.json.Json;
import org.reactivestreams.Publisher;
import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;

import org.springframework.core.ResolvableType;
import org.springframework.core.codec.AbstractDecoder;
import org.springframework.core.codec.StringDecoder;
import org.springframework.core.io.buffer.DataBuffer;
import org.springframework.http.MediaType;
import org.springframework.util.ConcurrentReferenceHashMap;
import org.springframework.util.MimeType;

/**
 * Decode a byte stream into JSON and convert to Object's with
 * <a href="https://github.com/Kotlin/kotlinx.serialization">kotlinx.serialization</a>.
 *
 * <p>This decoder can be used to bind {@code @Serializable} Kotlin classes.
 * It supports {@code application/json} and {@code application/*+json} with
 * various character sets, {@code UTF-8} being the default.
 *
 * <p>Decoding streams is not supported yet, see
 * <a href="https://github.com/Kotlin/kotlinx.serialization/issues/1073">kotlinx.serialization/issues/1073</a>
 * related issue.
 *
 * @author Sebastien Deleuze
 * @since 5.3
 */
public class KotlinSerializationJsonDecoder extends AbstractDecoder<Object> {

    private static final Map<Type, KSerializer<Object>> serializerCache = new ConcurrentReferenceHashMap<>();

    private final Json json;

    // String decoding needed for now, see https://github.com/Kotlin/kotlinx.serialization/issues/204 for more details
    private final StringDecoder stringDecoder = StringDecoder.allMimeTypes(StringDecoder.DEFAULT_DELIMITERS, false);

    public KotlinSerializationJsonDecoder() {
        this(Json.Default);
    }

    public KotlinSerializationJsonDecoder(Json json) {
        super(MediaType.APPLICATION_JSON, new MediaType("application", "*+json"));
        this.json = json;
    }

    @Override
    public boolean canDecode(ResolvableType elementType, MimeType mimeType) {
        return super.canDecode(elementType, mimeType) && (!CharSequence.class.isAssignableFrom(elementType.toClass()));
    }

    @Override
    public Flux<Object> decode(Publisher<DataBuffer> inputStream, ResolvableType elementType, MimeType mimeType, Map<String, Object> hints) {
        return Flux.error(new UnsupportedOperationException());
    }

    @Override
    public Mono<Object> decodeToMono(Publisher<DataBuffer> inputStream, ResolvableType elementType, MimeType mimeType, Map<String, Object> hints) {
        return this.stringDecoder
                .decodeToMono(inputStream, elementType, mimeType, hints)
                .map(jsonText -> this.json.decodeFromString(serializer(elementType.getType()), jsonText));
    }

    /**
     * Tries to find a serializer that can marshall or unmarshall instances of the given type
     * using kotlinx.serialization. If no serializer can be found, an exception is thrown.
     * <p>Resolved serializers are cached and cached results are returned on successive calls.
     *
     * @param type the type to find a serializer for
     * @return a resolved serializer for the given type
     * @throws RuntimeException if no serializer supporting the given type can be found
     */
    private KSerializer<Object> serializer(Type type) {
        KSerializer<Object> serializer = serializerCache.get(type);
        if (serializer == null) {
            serializer = SerializersKt.serializer(type);
            serializerCache.put(type, serializer);
        }
        return serializer;
    }
}
